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Some key aspects of the behavior of graphene nanoribbon (GNR) FETs for high-frequency analog applications are identified and discussed by means of a simulation study based on a full-quantum ballistic transport model. GNRs of width in the order of 10 nm are considered, where the small band-gap and the consequent leakage currents due to band-to-band-tunneling (BTBT) require a careful design. Simulations performed with a realistic model for source/drain metal contacts indicate that a proper choice of the drain doping profile can partially suppress BTBT currents. A 40-nm gate-length 2-nm SiO2 gate-dielectric GNR-FET can achieve a peak small-signal voltage gain of about 30 and a cut-off frequency well above 1 THz.

In this paper, we present a mode space method for atomistic non-equilibrium Green's function simulations of armchair graphene nanoribbon FETs that includes electron-phonon scattering. With reference to both conventional and tunnel FET structures, we show that, in the ideal case of a smooth electrostatic potential, the modes can be decoupled in different groups without any loss of accuracy. Thus, inter-subband scattering due to electron-phonon interactions is properly accounted for, while the overall simulation time considerably improves with respect to real-space, with a speed-up factor of 40 for a 1.5-nm-wide device. Such factor increases with the square of the device width. We also discuss the accuracy of two commonly used approximations of the scattering selfenergies: the neglect of the off-diagonal entries in the mode-space expressions and the neglect of the Hermitian part of the retarded self-energy. While the latter is an acceptable approximation in most bias conditions, the former is somewhat inaccurate when the device is in the off-state and optical phonon scattering is essential in determining the current via band-to-band tunneling. Finally, we show that, in the presence of a disordered potential, a coupled mode space approach is necessary, but the results are still accurate compared to the real-space solution.

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